An Action in Omnilert is a pre-defined message for use as part of a Scenario.
Of course, a Scenario can launch one or more actions to send out multiple messages to various endpoints, all with a single click or using the Scenarios app for iPhone/Android.
So, you can think of your actions as the building blocks for a Scenario. Some scenarios might only need one "action" (e.g. sending a text to every subscriber), while others might use several actions to send different messages to different groups or endpoints.
Since an action is a saved message, the possible uses are nearly endless. Any message you want to automate will use actions to do it.